Urea is a well-known fertilizer and industrial chemical which is useful for many purposes, including urea-formaldehyde resins and glues. Urea itself is a highly water-soluble salt, and is relatively easy to store, transport and handle. Urea does not have any perceptible odor and is not classified as a hazardous chemical. By contrast, ammonia or ammonia compounds give off a very strong ammonia odor in water solutions. Ammonia is classified as a hazardous chemical, and is much more difficult to store, transport and handle, compared with urea. However, in many cases, ammonia is preferred for use in chemical processes instead of urea, because of faster reaction times, fewer objectionable byproducts, and/or higher utilization efficiency. For example, in SNCR (Selective Non-Catalytic NOx Reduction) processes, which involve injecting a chemical such as ammonia or urea into NOx-containing flue gases at temperatures from 1600 to 2000 F., it has been determined that ammonia reacts more quickly than urea and produces lower concentrations of undersirable byproducts such as N.sub.2 O, CO, or NH.sub.3 slip, compared with urea under otherwise identical SNCR process conditions. It is therefore desired to combine the advantages of both urea and ammonia into process systems utilizing ammonia.
One of the objectives of the present invention is to provide an effective and inexpensive means to use urea for storage, transportation, and handling as part of an ammonia process system. The present invention accomplishes this by rapid conversion of stored urea into ammonia for continuous use during long operational periods, whereby only very small amount(s) of ammonia are actually present in the equipment components of the ammonia process system. The cost and difficulty of providing storage, transportation, and handling of ammonia can therefore be avoided without sacrificing the performance benefits of using ammonia in the ammonia process system.
There are many examples which illustrate the advantages of the present invention. One such example might be for NOx control applications in large trucks powered by Diesel engines, where urea can be safely stored in the truck, but rapidly converted to ammonia at required flowrates for use in NOx control equipment to remove NOx pollution from the Diesel engine exhaust gas. It would be difficult to imagine that regulatory agencies would permit storage of ammonia (a hazardous chemical) in a large number of such trucks. In the event of a traffic accident, many innocent bystanders could be injured by ammonia fumes. However, odor-free and safely-stored urea chemical is not expected to present a difficult permitting problem for the same application. A reliable and inexpensive means for rapidly converting a given flowrate of aqueous urea into aqueous ammonia would provide a means for implementing this type of NOx control system into large trucks powered by Diesel engines. Other examples could be described, such as storage of urea for powerplant boiler NOx control systems, where aqueous urea is converted to aqueous ammonia prior to injection into the NOx-containing flue gas. As another example, the present invention could be used to provide rapid recovery of NH.sub.3 and CO.sub.2 from urea-containing water streams, as an improvement to urea manufacturing processes. Many other examples could also be cited.
A literature review indicates that no such rapid urea conversion process is commercially available. One of the best texts is "Urea, Its Properties and Manufacture" by G. Tsei-Yu Chao, Library of Congress Catalog Card No. Ai-11254, 1967. Chapter III-2 (Hydrolysis of Urea, pages 97-118) and Chapter III-7 (Miscellaneous Reactions of Urea, pages 177-194) make no mention of the use of catalysts to promote the conversion of urea. The text does show that 20% to 26% urea conversion (depending on addition of acids or alkalis) can be accomplished at 212.degree. F. in 60 minutes residence time at atmospheric pressure. Such long reaction times with such low urea conversion percentages are not of interest.
Next, the U.S. Pat. Nos. of L. P. Schell (4,087,513 and 4,168,299 and 4,220,635) relating to urea hydrolysis were investigated. The '513 patent involves urea hydrolysis combined with absorption and condensation of mixtures of carbon dioxide and water vapor. The '299 (continuation) patent involves hydrolyzing urea to carbon dioxide and ammonia using vanadium pentoxide catalyst to achieve about 30% urea conversion at 215.degree. F. in 30 minutes at atmospheric pressure. The '635 (continuation) patent shows the effectiveness of a variety of vanadium compounds. The best results were 58% urea conversion with 0.19% vanadium at about 220.degree. F. in 360 minutes, and 100% urea conversion with 0.30% vanadium at the same temperature in 840 minutes. Both the '299 and '635 patents specify that reaction times of "at least 10 minutes, preferably about 15 to 60 (360) minutes, is desirable for most applications." One of the main objectives of the present invention is to achieve significant levels of urea conversion in less than 10 minutes reaction time.
Further investigation of urea manufacturing processes revealed the common practice of hydrolyzing urea contained in waste water streams to ammonia and CO.sub.2 for recovery and re-use. This is accomplished by feeding a 1% or 2% aqueous urea solution at 300 psig into a steam-sparged reactor to achieve 99.9+% urea conversion at 400.degree. F. in about 60 minutes reaction time. This is commonly practiced in urea manufacturing plants, and no catalyst is used. Again, the long reaction time which is required is a significant disadvantage.
Finally, a search was conducted to determine if a catalyst to promote rapid urea conversion is taught in the prior art. U.S. Pat. No. 4,124,629 describes urea hydrolysis which occurs in the presence of alumina and one or more of the iron group metals. In this case, co-precipitation of nickel and alumina from solution by urea hydrolysis was shown to produce a more stable catalyst than by prior methods. The discussion in Col. 6, Example II indicates that with a large excess of catalyst at 210 F. "rapid evolution of CO.sub.2 occurred due to urea hydrolysis . . . . After about 4 hours, the pH had risen from 2.3 to 4.5 . . . . Urea hydrolysis was allowed to continue, the pH rising to 5.3 in 85 more minutes, where it remained for about 2 hours . . . " This teaching indicates that even with a large excess of catalyst, urea conversion reaction times are a matter of hours, not minutes. U.S. Pat. No. 4,018,769 entitled "Urea Cyanurate Manufacture" describes (at the top of Col 4) urea hydrolysis, where the effect of acid and temperature in mixtures of urea and elemental sulfur are said to accelerate reaction rates, with 90 C. to 150 C. (or 190 F. to 300 F.) being the upper limit of temperatures desired for urea cyanurate conversion, beyond which urea loss by hydrolysis exceeds acceptable limits. This teaching is consistent with the commercial practice of urea hydrolysis at 400 F. in 60 minutes, but it is not known whether the presence of elemental sulfur, as taught, hinders or helps the urea hydrolysis reaction rate. The present invention seeks to avoid the use of additional additives such as elemental sulfur.
It appears that neither prior art nor presently-practiced urea manufacturing processes utilize or anticipate the present invention using catalyst materials to achieve high percentages of urea conversion in very short reaction times.